Respirometers have been commonly used to determine the metabolic characteristics of cellular suspensions by measuring oxygen requirements during respiration. Measurement of respiration provides the observer with information about the metabolic process which is not available by any other technique. In the biochemical reactions that describe the mechanism of metabolism, utilization of oxygen is intimately related with the energy generation capability of the living system. For this reason measurement of respiration has been used as an indicator of the state of health of a living organism. These measurements are now used in the fields of biomedicine, water pollution, fermentation, and composting on a routine basis as well as in research work.
In the past, respirometers were primarily used in research or diagnostic studies. Small volumes of microorganism cultures were used and the respirometers were manually operated and manually read. Today large volume respirometers, such as those disclosed in U.S. Pat. Nos. 3,348,409; 4,314,969 and 3,740,320, are being used with large samples of cultures obtained from bio-conversion processes such as wastewater treatment, fermentation, and composting. They are also used with small animals and fish. These instruments are automatically operated and are made in laboratory, on-line, and submersible models. The size and varied use of these units has created problems in temperature control, in cleaning, and in maintaining an airtight chamber.
A respirometer in use today for small culture work is known as the Warburg respirometer. The Warburg respirometer consists of a small glass sample flask having a well containing a caustic solution such as KOH. The microorganism culture, suspended in water, is introduced to the flask, KOH is added to the well, and the air space above the suspension is connected to a U-tube manometer. The entire system is mechanically shaken to accelerate oxygen transfer from the air space into the liquid. As microorganisms respire, there is a decrease in the amount of oxygen dissolved in the water and additional gaseous oxygen from the air space is dissolved in the suspension due to mass transfer. Respiration also results in an increase in dissolved carbon dioxide which is transferred from the dissolved state to the gaseous state and is absorbed by the KOH in the well. The net effect is a decrease in the partial pressure of oxygen. The change in pressure is measured by the change in liquid level in a glass U-tube manometer. By utilizing certain calibration procedures, the reading can be interpreted as oxygen utilization in volume or weight units. While the Warburg respirometer is satisfactory for a research environment, it is not appropriate for routine use in connection with bioconversion processes. It is fragile, has a small sample size and is too complicated for automatic operation.
In U.S. Pat. No. 3,348,409 entitled "Method and Apparatus for Analyzing Gas Absorption and Expiration Characteristics," there is described a recording respirometer which measures the rate of oxygen utilized by a respiring culture. This apparatus consists of a large volume sample chamber which contains the culture, a separate carbon dioxide scrubber, an electronic oil manometer (gas volume transducer), and an air pump all connected by tubing. The air pump continuously forces air through the sample chamber to mix and aerate the sample. The air is forced through the scrubber for carbon dioxide absorption, and through the electronic manometer where the change in partial pressure of oxygen is measured. The signal from the electronic manometer is fed to a strip chart recorder to continuously record the oxygen utilization. By using large volume sample chambers and by automating the measurement, this structure did solve certain major disadvantages of the Warburg respirometer. The separation of components and required tubing connections, however, have resulted in problems of maintaining an airtight system and in temperature control. The use of this prior structure with animals, fish, and plants has accentuated the problem of temperature control because to achieve good control it is necessary to place the entire apparatus in a large environmental chamber. Temperature control using such a chamber is not as efficient as using a temperature; controlled water bath.